1. Field of the Invention
The present invention relates in general to shipment of dry chemicals. More particularly, the present invention relates to an improved system for unloading dry chemicals from a liner within an intermodal container.
2. Description of the Related Art
Dry chemicals are sometimes shipped in intermodal containers. The containers are referred to as "intermodal" because they can be shipped via different modes, such as truck, rail car and ocean liner. More particularly, the intermodal containers may be placed on a chassis to form a trailer which can be pulled behind a tractor. That is, the container plus the chassis form the trailer. Of course, when the container is shipped on sea, the chassis is not attached. To ship the dry chemicals in the container, a liner bag is inserted into the container. The liner bag may be a polymer liner and formed of a material such as polyethylene. The liner can be recycled. The dry chemicals may be in the form of a powder or pellets. An example of a powderous chemical is purified terephthalic acid (PTA). An example of a pellet form chemical is polyethylene terephthalate (PET). To fill the container, there are two openings at the top and back of the liner. Dry chemical is blown into the liner through one of the openings in the liner. Dry chemical may be blown with an inert (perhaps nitrogen) gas. The inert gas is vented from the container through the other opening in the liner. Any residual dry chemical is then filtered from the inert gas before venting.
Hereinafter, the end of the container which would be positioned away from the driver, toward the doors is referred to as the "back" of the container. The dry chemical is loaded and unloaded from the container with the doors of the container open. With the doors open, the liner must be supported by some means, from the back of the container. Conventionally, wood on metal supports were built into the container every time a new liner was to be installed. The supports were in the form of a framed bulk head. To remove the liner, the supports were completely dismantled. Building the bulk head was time consuming and the strength of the bulk head could vary.
To unload the chemical from the container, an outlet is located at the back bottom of the liner. The outlet may be attached to a conduit leading to unloading equipment. Various make-shift approaches, some more successful than others, were used to attach the outlet to the unloading equipment. The container was then tilted, either on its own power or by tilting a ramp on which the trailer was parked. The container could be tilted up to about 45.degree.. The dry chemical may weigh up to 50,000 pounds. For safety reasons, it may be important that a substantial amount of the dry chemical be removed from the liner before the container reaches the 45.degree. tilt. The weight may be too great to be supported by the hand-made bulk head.
One characteristic of dry chemical powders, and all powders for that matter, is the "angle of repose." The "angle of repose" is the steepest angle which the material can form under a given set of conditions. For example, if a powder is poured onto a flat surface, one angle of repose is the angle the resulting mound on the flat surface makes with the flat surface. Generally speaking, the larger the angle of repose, the more difficult it is to achieve flow. For example, if a powder is poured onto a flat surface, next to a ledge and the powder has a small angle of repose, the powder will simply flow off of the ledge. On the other hand, if the powder has a large angle of repose, the powder may form a steep mound next to the ledge, and will not flow off of the ledge. The angle of repose varies from powder to powder. Moreover, in a single powdered chemical, such as PTA, the angle of repose varies from manufacturer to manufacturer.
In the conventional system for transporting and unloading powdered chemicals, the liner conformed to the shape of the container and thus was substantially box-shaped. FIG. 1 is a cross-sectional view of a conventional liner, taken from a plane parallel to and below a top of the liner. The conventional liner had an outlet 2 for unloading the chemical. The outlet is at the bottom middle of the back of the container. Even when the container was completely tilted (FIG. 1 represents a tilted container), powder remained in the liner, extending from ledges 2a of the outlet 2 to the trailer wall 4. The height H to which the powder remains on the wall (depth in a horizontal container) is determined by the angle repose.
Furthermore, in the conventional system, there was not an immediate drop-off after the outlet. That is, when the container was horizontal, the outlet emptied into a conduit which was horizontal for a short distance d before becoming vertical to drop off the back of the container. FIG. 2 is a cross-sectional view of the conventional liner, taken through line II--II of FIG. 1. FIG. 1 shows the liner as being substantially empty and tilted, whereas FIG. 2 shows the liner as being substantially full and horizontal. As can be seen from FIG. 2, when the container was horizontal and the powder had a relatively large angle of repose, there was no discharge of powder. As discussed above, the powder must flow before the container is fully tilted. FIG. 3 is the same view as FIG. 2, but with the container and the liner inclined at an angle with a horizontal to show how the powder flows. As can be seen in FIG. 3, after tilting, powder began to flow. However, a portion P of the outlet was not being used. Thus, the effective diameter of the outlet was reduced by the portion P and discharge of powdered chemical was slow. The buyer of the powdered chemical may require large quantities of the chemical. However, with the conventional transporting and unloading system, unloading the chemical from the liner could require up to six hours. This time increased receiving costs, limited the rate at which the buyer could receive the chemical and thus limited production in the buyer's plant.